This invention relates to an electrode and separator assembly for galvanic cells. This invention particularly relates to coiled, i.e., so-called jelly roll type, galvanic cells and particularly to the electrode and separator assembly therefor.
Jelly roll type galvanic or electrochemical cells, e.g. nickel cadmium cells, have the electrodes electrically separated by an insulative sheet separator, the two electrodes and the separator being rolled up to fit into a cylindrical container An electrical lead extends from one of the electrodes to serve as a terminal, to connect to an exterior load. The container engages the other electrode directly or through another lead to serve as the other terminal. Such a cell is shown, for example in U.S. Pat. No. 3,900,340.
A significant advantage of using the jelly roll construction is that the interfacial surface area between the two electrodes is greatly increased relative to the interfacial surface area obtainable, for example, when using a bobbin construction. However, as the surface areas of the electrodes increase, the amount of separator required increases proportionately. The net result is that a significant percentage of the internal volume of the cell is occupied by the separator. The more space that is required for the separator, the less that is available for active electrode material.
Cell constructions used in commercially available batteries and/or disclosed in literature exemplify numerous methods of using separators to physically isolate one electrode from the other electrode. Most of these use separator material that is longer and wider than one of the electrodes. In many cases, the amount of excess separator exceeds the minimum amount of required separator by at least 15 percent, based on the surface area of the electrode, and often up to or greater than 30 percent. Even with this excessive amount of separator, many cell constructions have internal electrical shorting caused by loose mix particles.